4-3-Z Handshake Lines; Data Lines - Keithley 776 Instruction Manual

DA"
SOURCE
VALID
I
NAFO
I
ACCEPTOR
I
I
I
I
NDAC
I
ACCEPTOR
,
I
I
DATA DATA
TRANSFER
TRANSFER
BEGINS
ENDS
Figure
4-2. IEEE Handshake Sequence
4-3-Z. Handshake Lines
The bus uses three handshake lines that operate in an
interlocked sequence. This method assures reliable data
transfer regardless of the transfer rate. Generally, data
transfer will occur at a rate determined by the slowest
active device on the bus.
One of the handshake lines is controlled by the
data
source, while the remaining two lines arc controlled by
accepting devices. The three bus handshake lines arc:
1. DAV
(Data Valid) - The source controls the state of
the DAV line.
2. NRFD
(Not Ready For Data) - the acceptor controls
the state of the NRFD line.
3.
NDAC
(Not Data Accepted) - the acceptor also con-
trols the NDAC line.
The complete handshake sequence for one data byte is
shown in Figure 4-2. Once data is on the bus, the source
checks to see that NRFD is high, indicating that all
devices on the bus are ready for data. At the same time
NDAC should be low from the previous byte transfer. If
these conditions are not met, the source must then wait
until the NRFD and NDAC lines have the correct status.
If the source is controller, NRFD and NDAC must re-
main stable for at least 100 ns after ATN is set low:
Because of the possibility of bus hang up, some control-
lers have time-out routines to display error messages if
the handshake sequence stops for any reason.
Once the NRFD and NDAC lines are properly set,
the source sets the DAV line low, indicating that dataon
the bus is now valid. the NRFD line then goes low; the
NDAC line goes high once all devices on the bus have
accepted the data. Each device will release the NDAC
line
at its own rate, but the NDAC line will not go high
until the slowest device has accepted the data byte.
After
the
NDAC line goes high, the source then sets
the DAV line high to indicate that the data on the bus is
no longer valid. At this point, the NDAC line returns to
its low state. Finally, the NRFD line is released by each
of the devices at their own rates, until the NRFD line
finally goes high when the slowest device is ready, and
the bus is set to repeat the sequence with the next data
byte.
The sequence just described is used to transfer both
data and multiline command. The state of the ATN line
determines whether the data bus contains data or com-
mands.
4-3-3. Data Lines
The IEEE-488 bus uses the eight data lines that allow
data to be transmitted and received in a bit-parallel,
byte-serial manner. These eight lines use the convention
4-3